The present invention relates to a battery power circuit for use in a motorcar or the like, which circuit outputs a plurality of voltage levels.
FIG. 15 shows a related-art automobile battery power circuit disclosed in TOYOTA Technical Review Vol. 50 No. 1 Jun. 2000 p.37 or the like. In FIG. 15, the reference numeral 7 represents an engine; 6, an electric motor for transmitting power to the engine through a first power transmitter 8; 5, a power conversion circuit for transmitting electric power to the electric motor 6; and 200, a battery group constituted by a plurality of batteries connected in series, connected to the power conversion circuit 5, and typically having a rated voltage of 36 V. The reference numeral 90 represents a load B connected to the battery group 200; 300, a DC/DC converter for stepping the voltage of the battery group down; 100, a battery charged by the DC/DC converter 300, and typically having a rated voltage of 12 V; and 4, a load A connected to the battery 100. Typically the load A is called a 14V load and the load B is called a 42V load. The reference numeral 9 represents a key switch for initial start-up; and 10, a cell motor for rotating at the time of initial start-up and transmitting power to the engine through a second power transmitter 11. Here, each of the first and second power transmitters 8 and 11 may include a power transmission unit for connecting the electric motor 6 and the engine 7 or the cell motor 10 and the engine 7 directly through a shaft, indirectly through a belt, or indirectly through gears.
First, the operation will be described with reference to FIG. 15. The power conversion circuit 5 converts the voltage of the battery group 200 into an alternating current or an intermittent direct current so as to rotate the electric motor 6 and start the engine 7. Generally, it is known broadly that useless fuel consumption during temporary stop (waiting at stoplights, a traffic jam, etc.) while driving a motorcar can be suppressed by stopping the engine 7 during the period of the temporary stop and starting the engine 7 again when the necessity of restarting arises (idle stop operation). At the time of this restart, however, power required for starting the engine 7 has to be supplied in a short time in order to restart the engine 7 smoothly by depressing an accelerator pedal or the like, and further to lead the motorcar to start its motion. To attain the supply of the required power, such a configuration is proposed that a voltage of 36 V is retained by the battery group 200, and a large current flows into the electric motor 6 through the power conversion circuit 5 so that high instantaneous power is transmitted to the electric motor 6. On the contrary, during the rotation of the engine 7 with fuel, the power of the engine 7 is transmitted to the electric motor 6 so that electricity is generated. Thus, the battery group 200 is charged to 36 V through the power conversion circuit 5.
However, most of electric components mounted on motorcars are formed of 14 V systems. Therefore, a battery whose rated voltage is 12V is required unless all the electric components are replaced by 42V systems. Not to say, in the present circumstances where there are few electric components supporting 42V systems and these electric components are expensive (due to the problem of quantity), it can be foreseen that electric components support 14V systems fundamentally, and hence it can be foreseen that the 14V load has a high capacity.
Therefore, a high-capacity stationary battery 100 is charged with a voltage of 12 V using the output of the battery group 200 through the DC/DC converter 300 so as to operate electric components. In addition, when electric components of 14V systems are used stationarily during driving or the like, the electric components have to be operated directly from the DC/DC converter 300. To this end, the DC/DC converter 300 has to have a high capacity.
Thus, even if a battery group having a rated voltage of 36 V is provided for attaining the idle stop operation, the power circuit will be expensive because the high-capacity DC/DC converter 300 or the high-capacity stationary battery 100 is required. Since the high-capacity DC/DC converter 300 generates a great deal of heat, accessories for cooling are required to thereby cause further increase in cost.
In addition, it is also generally known that greater power is required for starting the engine 7 at the time of initial start-up of the engine 7 (start-up in the state where the temperature of the engine 7 is low, for example, when driving is started initially). In order to use the electric motor 6 to perform initial start-up, it is necessary to supply a higher current to the electric motor 6. To this end, the power conversion circuit 5 has to have large-current specifications. Thus, the power conversion circuit 5 becomes expensive.
As a solution to this problem, there is proposed a system in which initial start-up is performed by the rotation of a cell motor based on key operation while only the idle stop operation is performed by an electric motor. By this system, the power conversion circuit does not have to have large-current specifications. However, at the time of initial start-up in such a configuration, it is necessary to supply a large quantity of charges to the cell motor. Thus, a high-capacity stationary battery is still essential. Therefore, even if electric components of 14V systems are reduced in the future, a high-capacity stationary battery still has to remain. This causes a great obstacle to reduction in cost.
In a related-art power circuit for an electric motorcar, a high-capacity DC/DC converter is required for obtaining 12 V from 36 V as described above. Accessories associated with the DC/DC converter are expensive, and the high-capacity 12V stationary battery 100 is also expensive and becomes an obstacle to miniaturization.